The present invention relates generally to feed-through filters utilized to separate unwanted interference from a signal path. More particularly, the invention relates to feed-through filters utilized in implantable medical devices, such as heart pacemakers and the like.
Heart pacemakers and other implantable medical devices are constructed having an outer housing in which the necessary electronic components are contained. The outer housing must be formed of a material which is compatible to be placed inside the human body. It is also important to shield the electronics within the housing from external sources of electromagnetic interference (EMI). Titanium is often utilized to satisfy these dual requirements of shielding and biocompatability.
At least one elongate lead will generally extend from the electronics within the outer housing to a desired location inside the body. While the outer housing may shield the internal electronics from direct EMI radiation, steps are also taken to inhibit transmission of EMI along the lead itself. For example, selected capacitive and/or inductive components may be mounted on a circuit board along with the other internal electronics to provide EMI filtering. Alternatively, a feed-through filter may be provided at the location where the elongate lead passes through the outer housing. Such a feed-through filter is shown and described in U.S. Pat. No. 4,424,551, incorporated herein by reference.
Although pacemaker signals are relatively low voltage, capacitors utilized in feed-through filtering arrangements must often be constructed to withstand relatively high voltage levels. This is to ensure that the capacitor does not become damaged if subjected to voltage transients, such as those caused by undesirable defibrillation pulses or the like. Generally, however, a capacitor having a higher voltage rating will be larger than a lower voltage capacitor of the same capacitance value. In view of the general trend toward miniaturization in electronic devices, larger components are often considered to be undesirable.